This invention relates generally to radio repeater systems. More specifically this invention pertains to systems wherein a plurality of mobile or portable stations communicate with one another over several communications channels through repeaters operating on each of such channels. Even more specifically, this invention is directed to arrangements for establishing communication between a first radio station and a second radio station through a repeater and over a radio channel selected from a plurality of such channels.
In many localities, a plurality of land mobile radio frequency channels are allocated for communication among vehicles. Generally, there may be, and usually are, many more vehicles having mobile radio stations than there are available radio channels in a given locality. Each of these radio channels usually includes at least two separate and distinct frequencies, one for transmitting and the other for receiving.
Consequently, some arrangement is needed for enabling a mobile vehicle user to obtain access to a radio channel that is not in use and to thereafter establish communication with another mobile vehicle user over that radio frequency channel.
One such system is disclosed by J. C. Berti et al in U.S. patent application Ser. No. 956,085 filed Oct. 30, 1978 and entitled IMPROVED ARRANGEMENT AND METHOD FOR ESTABLISHING RADIO COMMUNICATION IN A SYSTEM, now abandoned in favor of continuing application Ser. No. 110,995, filed Jan. 10, 1980 by Berti et al, now abandoned. In the Berti system, each mobile or portable radio station (including a transmitter and receiver) can be placed in an idle or call originate mode. When a mobile station is placed in the call originate mode, its receiver scans all of the allocated radio frequency channels in the system until an unused channel is found. When an unused channel is found, the station's transmitter and receiver become operable on that channel. The mobile station's transmitter sends a busy signal to the repeater operating on the unused channel. The repeater receives this busy signal and transmits a busy signal identical to the busy signal receiver. When the mobile station that sent the busy signal receives it back from the repeater, it sends out a group signal identifying a particular subset of mobile or portable radio stations with which the user desires to communicate. The group signal is re-transmitted by the repeater and is recognized by each radio station within that subset and they are activated on the channel selected by the busy tone exchange.
In the idle mode, the receiver of each mobile or portable station in the system scans all of the allocated radio frequency channels for its predetermined group signal. When the receiver detects its predetermined group tone or tones on a channel, it stops scanning, and enables the transmitter and receiver on that channel for communication through the repeater with the station originating the call. There may be a plurality of such stations associated with a predetermined group tone or tones.
The Berti system works well under most circumstances. However, under certain circumstances, a mobile radio station can erroneously lock onto a channel that is non-vacant. When a mobile station is placed in the call originate mode, it scans all of the channels for the presence of a busy signal. If this scanning takes place during a fade situation or when for any other reason the busy signal detector of the call originating station has failed to detect the busy tone, the call originating station may lock onto a channel that is non-vacant and it will begin to operate on that channel.